1. Field of the Invention
The present invention relates to an optical fiber, and, in more detail, relates to a polymer-cladding optical fiber to be used for a double-cladding fiber having two cladding layers.
2. Description of the Related Art
Conventionally, in a double-cladding fiber transmitting excitation light in a cladding layer for exciting signal light to be transmitted in a core, small refractive index resin coatings the outside of a glass cladding as a second cladding layer. As the small refractive index resin, fluorine-based UV-curable resin is used for making a refractive index smaller than that of a first cladding layer for transmitting the excitation light.
Meanwhile, in Published Japanese Translation of PCT International Application Publication No. 2006-519495, a multi-cladding structure by using silica glass for the second cladding layer and doping fluorine into this silica glass to make a refraction index smaller than that of the first cladding layer is obtained. Further, at the periphery of the second cladding layer, a protection layer made of polymer is formed for protecting the second cladding layer of the silica glass from the outside.
In the configuration of the double-cladding fiber disclosed in Published Japanese Translation of PCT International Application Publication No. 2006-519495, the first cladding layer functions as a core for the excitation light propagation and the second cladding layer, which is made of fluorine-doped silica glass, functions as a cladding for the excitation light propagation. In Published Japanese Translation of PCT International Application Publication No. 2006-519495, the existence of the second cladding layer made of glass reduces a leak of an evanescent field from the excitation light propagating in the first cladding layer into the polymer protection layer and suppresses damage of the polymer protection layer by the evanescent field. Further, the second cladding layer doped with fluorine increases the heat resistant strength of the double-cladding fiber (multi-cladding fiber).
However, the NA of the multi-cladding fiber such as the double-cladding layer is proportional to a refractive index difference of the first cladding layer made of silica-glass and the second cladding layer made of silica-glass. Since there is a limit in a fluorine amount which can be doped into the silica glass, it is difficult in Published Japanese Translation of PCT International Application Publication No. 2006-519495 to greatly reduce the refractive index of the second cladding layer by the fluorine doping.
On the other hand, the fluorine content amount can be increased in a resin such as a UV-curable fluorine-based resin more than in the silica glass. Accordingly, a polymer cladding as the second cladding layer provides a higher possibility of increasing the NA of the fiber. For an application requiring an increase in an output light intensity such as a fiber laser, it is required to transmit more excitation light within the first cladding and thus the polymer cladding, which can be expected to provide a larger NA, is used for the second cladding layer (refer to Japanese Patent Application Laid-Open Publication No. H10-197731).
However, there is a problem that the fluorine-based resin has a weak mechanical strength compared to non-fluorine-based UV-curable resin. Accordingly, when using the fluorine-based resin for the second cladding layer (first coating layer) of the double-cladding fiber as in the technique disclosed in Japanese Patent Application Laid-Open Publication No. H10-197731, it is necessary to coat the second cladding layer with a second coating layer which is superior in the mechanical strength, which results in a higher cost.
Further, when an incident light intensity changes as in the fiber laser which uses a double-cladding fiber disclosed in Japanese Patent Application Laid-Open Publication No. H10-197731, self heating of the fiber is enhanced and the mechanical strength of the polymer cladding material is reduced. In particular, when the fiber laser has a high output, that is, when the temperature is increased, the mechanical strength is reduced in the second cladding layer (first coating layer) which is made of the fluorine-based resin. Accordingly, the protection function for the first cladding layer including silica glass as a main component and the protection function for the core are lost, which results in a failure of the fiber laser.
Moreover, in Published Japanese Translation of PCT International Application Publication No. 2006-519495, it is inevitably necessary to configure the second cladding layer using silica glass for a technical purpose of protecting the resin protection layer from the evanescent field leaked from the first cladding layer. As described above, since the amount of the fluorine doped into the silica glass is limited to a small value, the reduction of the refractive index by the fluorine doping cannot be expected in a large extent and it is difficult to increase the NA.